Serological studies confirmed that B/Beijing/184/93-like viruses have undergone marked antigenic changes from the previous vaccine strain. The data were used by the USPHS and WHO to recommend inclusion of a new strain in inactivated influenza vaccines for the 1999-2000 season. Our laboratory produced reference reagents for this strain to facilitate qualification and release of approximately 90 million doses of influenza vaccine for the United States. In addition, as a reference strain for production purposes, we cloned a reassortant influenza virus strain (RESVIR 14) with increased ability to replicate in eggs, and this strain was distributed to WHO Influenza Centers, national laboratories and manufacturers. In response to the appearance of a new avian H9 subtype of influenza A virus in man, we have begun preparations for the possibility of a pandemic if global spread of the strain should occur. We have begun work to produce a reference antiserum against this and similar avian strains, and we are engaged in efforts to produce high growth reassortants of these viruses which could be used for manufacturing purposes. We continued to examine the interaction between influenza virus matrix (M1) protein and ribonucleocapsids (RNP). We previously demonstrated that the binding to RNP of M1 proteins from high-growth strains (e.g. A/Puerto Rico/8/34 [PR8])is more resistant to disruption by higher salt concentration or lower pH compared to low-growth wild-type strains (e.g. A/Nanchang/933/95). In further investigations of attenuated influenza viruses (including some with ts lesions), we have found that the binding to RNP of M1s from attenuated strains is more readily disrupted by high salt or lower pH. The data add further evidence that the RNP binding domains of M1 that we have identified (Ye et al., J Virol 1999;73:7467) are central to replication and virulence of influenza viruses. Since replication and virulence may be affected by other viral proteins, we have cloned and expressed (in vitro and in vivo) the viral genes of PR8 including PB1, PB2, PA, NP, NA and NS. The role of these proteins in replication and attenuation is being investigated.